We propose a 14-band k·p model to calculate spin-orbit splittings of the valence subbands in semiconductor quantum wells. The reduced symmetry of quantum well interfaces is incorporated by means of additional terms in the boundary conditions which mix the Γ15 conduction and valence Bloch functions at the interfaces. It is demonstrated that the interface-induced effect makes the dominating contribution to the heavy-hole spin splitting. A simple analytical expression for the interface contribution is derived. In contrast to the 4×4 effective Hamiltonian model, where the problem of treating the Vzkz3 term seems to be unsolvable, the 14-band model naturally avoids and overcomes this problem. Our results are in agreement with the recent atomistic calculations [Luo, Phys. Rev. Lett. 104, 066405 (2010)PRLTAO0031- 900710.1103/PhysRevLett.104.066405].